Update system.py

[foreign-sub]

Add
get_img_fbx_gw_back
get_img_fbx_gw_back_expansion

[foreign-sub]

Current implementation returns the full url when get_img_fbx_gw_back(...) is called, but once #22 is merged i think i'll change it to send back only a short url, then the user will reconstruct the full url with
f"{fbx.fbx_url}{fbx.system.get_img_fbx_gw_back(0)}"

edit : removed host_url